Shape memory alloys (SMAs) may exist as two phases, a lower modulus, lower temperature, crystalline martensite phase and a higher modulus, higher temperature, austenite phase of a different crystal structure. The transition from one phase to the other may, by appropriate choice of alloy system, alloy composition, heat treatment or applied stress, be selected to occur over a temperature span of from −100° C. up to about +150° C. or so. But many useful SMA alloys exist in their martensite form at, or slightly above, about 25° C. or so, and transform to their austenite form at temperatures ranging from about 60° C.-80° C. or so. These characteristics substantially assure that the SMA will be in its martensitic phase at essentially any ambient temperature experienced by a motor vehicle but may be readily transformed to austenite with only modest heating.
Shape memory alloys may be used as mechanical actuators. Commonly alloys for actuator applications are prepared as generally linear members. These members are commonly wires, but other suitable shapes include tapes, chains or cables. For brevity only, and without limitation, the term wire will be used in future sections. The wires, after shaping to a desired ‘remembered’ length or shape in their austenite phase are cooled to ambient temperature. On cooling the wires will revert to their martensite crystal structure. The wires may then be stretched and deformed to some predetermined length. The deformation exceeds the maximum allowable elastic strain which may be imposed on the actuator and is often termed pseudo-plastic deformation. These pseudo-plastically-deformed martensitic wires are in the appropriate starting condition for the actuator.
Generally the stretch or strain, that is, the change in length of the wire divided by its original or base length, applied during such pseudo-plastic deformation does not exceed 7% and more commonly may be 5% or less. Importantly, the base length, to which all length changes are referred, is the length of the wire in its high temperature, austenite, phase.
Deformed martensitic shape memory alloys may, when heated and transformed to austenite, revert to their original undeformed shape and are capable of exerting appreciable force as they do so. In changing shape, the wire will shorten by an amount substantially equal to the pseudo-plastic strain previously applied when it was in its martensitic form. So, by suitable choice of wire length, any desired displacement may be achieved. As an example, a 10 inch or so length of wire, prestrained to 5% strain, may enable a displacement of about one-half inch or so.
This change in length, in combination with the ability of the SMA to apply a significant force as it changes length, are the characteristics which make SMAs suitable for use as actuators in mechanical devices. In one common actuator design, a pseudo-plastically stretched martensite SMA wire of a length suitable for an intended displacement, is heated along its entire length and transformed to austenite. The transformation to austenite causing the wire to contract so that it may linearly displace an attached moving element.
In an exemplary application the attached moving elements may be an air dam which may be deployed, on-demand, by action of the SMA actuator. Of course, other linear motion devices such as latches may also be operated by SMA actuators. Also, by addition of pulleys and similar mechanical contrivances, an SMA actuator may be readily adapted to enable rotary motion. Any heat source may be used to elevate the SMA wire temperature and promote its transition to austenite. But, preferably, the wire should be heated uniformly along its length and throughout its cross-section so that substantially the entire volume may be heated and transformed, the transformation being effected generally simultaneously in the wire volume.
One convenient approach which assures generally uniform heating of the entire wire length is electric resistance heating. Electrical connections may be made to the SMA wire ends for attachment to a suitable power source, commonly a nominally 12 volt battery in the case of a motor vehicle, and a controlled current passed along the length of the wire. The applied current may be initially small and increased during the duration of the heating cycle using a ramp, sine, step or arbitrary variation of current with time or a fixed battery voltage may be applied and its heating power adjusted using pulse width modulation (PWM). Generally operation of the actuator occurs over a relatively short time period, typically on the order of 1 or 2 seconds. Application of power is generally under the control of a controller which may be independent of, or integrated with, other on-vehicle electronics. Many SMA-actuated devices are intended to operate over a fixed displacement. Thus, when the SMA device achieves its design displacement, the applied current is reduced to a value sufficient to maintain it at its design stroke. This end-of-stroke current may be termed a terminal current. Any suitable method may be used to signal the controller that end-of-stroke has been reached, including, for example, a contacting or non-contacting micro-switch. Once end-of-stroke is signaled, application of a continuing current sufficient to maintain the wire temperature is required. Suitable controllers and control strategies for accomplishing this are well known to those skilled in the art.
Actuator action may be reversed by stopping passage of the electric heating current and allowing the wire to cool to about ambient temperature and revert to its martensitic crystal structure. Generally forced cooling is neither required nor employed. During cooling, the SMA wire will not spontaneously change its length to its initial deformed length but, in its martensitic phase, it may be readily stretched again to its initial predetermined length. Any suitable approach, including deadweights, may be employed to stretch the wire, but often a spring positioned in series with the SMA wire is used. Stretching may be continued until the wire ends are positioned against preset stops which establish the predetermined wire length.
These changes in length result from the transition in crystal structure resulting from the imposed temperature changes. Provided the transition in crystal structure is fully reversible this cycle of extending and contracting the wire by application of suitable thermal stimulus may continue indefinitely.
In practice however, the phase transitions and the accompanying cyclic transitions from extended length to retracted length and back again to extended length, are not completely reversible. This irreversibility may lead to changes in the operating characteristics of the SMA wire with continuing use and even to fatigue of the SMA wire after extensive use. The occurrence of such fatigue may be promoted by overdriving or otherwise exceeding the design parameters or capabilities of the SMA wire
There is therefore a need to monitor SMA wire performance. In particular there is need for a method of detecting any change or deterioration in device capability. Preferably such change may be detected before it has increased to a level where a device may be rendered inoperative. More preferably the extent of any change may be interpreted to signal the remaining life of the device.